1. Field of the Invention
This invention relates to the control of the unique iridescence properties of solid films of nanocrystalline cellulose (NCC) particles prepared by sulfuric acid hydrolysis of cellulose, in particular to the control of the iridescence wavelength by means of mechanical energy input such as ultrasound or high-shear forces to the aqueous NCC suspension prior to film formation by evaporation.
2. Description of the Prior Art
Cellulose is the most abundant organic compound on earth. It is the structural component of the primary cell wall of higher plants and green algae, and it is also formed by bacteria, some fungi, and tunicates (invertebrate marine animals) [1].
Native cellulose has a hierarchical structure, from the polymeric glucose chains to the microfibrils which make up the cell walls of plants. The cellulose polymer chain is derived from D-glucose units, which condense through β(1→4)-glycosidic bonds giving a rigid straight chain having many inter- and intramolecular hydrogen bonds among the many glucosidic hydroxyl groups. These features allow the cellulose chains to pack closely to give areas of high crystallinity within the microfibril [2]. Cellulose microfibrils also contain amorphous regions randomly distributed along their length [3-5].
Cellulose whiskers or nanocrystals are obtainable by controlled acid hydrolysis of cellulose from various sources, in particular from wood pulp and cotton. The less-dense amorphous regions along the cellulose microfibril are more susceptible to acid attack during hydrolysis and cleave to give cellulose nanocrystals [6, 7]. Their low cost, renewability and recyclability, and their chemical reactivity allowing their chemical and physical properties to be tailored make nanocrystalline cellulose whiskers attractive for various applications [8, 9].
Nanocrystalline cellulose (NCC) is rodlike in shape with an aspect ratio which varies from 1 to 100 depending on the cellulose source. Wood cellulose nanocrystals average 180-200 nm in length with a cross section of 3-5 nm [9]. Nanocrystal dimensions also depend to a certain extent on the hydrolysis conditions used to obtain them.
The stability of NCC suspensions derives from sulfate ester groups imparted to the cellulose nanocrystal surfaces during hydrolysis with sulfuric acid. The NCC particles are therefore negatively charged in aqueous media and are thus electrostatically stabilized [7, 10-14]. Hydrochloric acid has also been used to produce NCC, but does not introduce charged surface groups [15].
The anisometric rod-like shape and negative surface charge of NCC particles result in suspensions which phase separate into an upper random phase and a lower ordered phase, at concentrations above a critical concentration, as described theoretically by Onsager [16]. The ordered phase is in fact a liquid crystal; liquid crystalline behaviour of cellulose suspensions was first reported by Rånby in 1951 [10]. Marchessault et al. and Hermans demonstrated that such suspensions displayed nematic liquid crystalline order [11, 17]. In 1992, Revol and co-workers showed that the suspensions in fact formed a cholesteric, or chiral nematic, liquid crystalline phase [12].
As shown in FIG. 1, chiral nematic liquid crystals contain rods arranged in pseudo-layers [18, 19]. The rods are aligned parallel to each other and to the plane of the layer, each layer being rotated slightly with respect to the layers above and below it, thereby producing a helix composed of the pseudo-layers. The pitch P of the helix is defined as the distance required for the NCC particles to make one full rotation about a line perpendicular to the layers. Between two critical concentrations, an NCC suspension will separate into two phases [16]. This region spans a range of approximately 3-8% (w/w) for cellulose nanocrystals. As the NCC concentration increases, the volume fraction of liquid crystalline phase increases until the suspension becomes completely chiral nematic above the upper critical concentration.
Aqueous NCC suspensions can be slowly evaporated to produce solid semi-translucent NCC films that retain the chiral nematic liquid crystalline order which forms above the critical concentration and increases in volume fraction as the water continues to evaporate. These films exhibit iridescence by reflecting left-handed circularly polarized light in a narrow wavelength band determined by the chiral nematic pitch and the refractive index of the film (1.55) according to Equation 1:λ=nP sin θ,  (1)
where λ is the reflected wavelength, n is the refractive index, P is the chiral nematic pitch, and θ is the angle of reflection relative to the surface of the film [20]. The wavelength reflected thus becomes shorter at oblique viewing angles. This reflectance was explained by de Vries [21] on the basis of Bragg reflections in a helicoidal arrangement of birefringent layers, as is the case for cellulose nanocrystals in a chiral nematic liquid crystal. When the pitch of the helix is on the order of the wavelengths of visible light (around 400 to 700 nm), the iridescence will be coloured and will change with the angle of reflection. It has been found that the iridescence wavelength can be shifted toward the ultraviolet region of the electromagnetic spectrum by increasing the electrolyte concentration (e.g., NaCl or KCl) in the NCC suspension prior to film formation [20]. The additional electrolyte partially screens the negative charges of the sulfate ester groups on the NCC surfaces, reducing the electrostatic repulsion. The rodlike particles therefore approach each other more closely, which reduces the chiral nematic pitch of the liquid crystal phase and shifts the iridescence to shorter wavelengths. This method of “blue-shifting” NCC film iridescence is limited by the amount of salt which can be added before the colloidal suspension is destabilized by too much screening and gelation occurs [13,20].
The NCC film iridescence colours observed by Revol et al. also depended on the cellulose source and the hydrolysis conditions (e.g., reaction time and ground cellulose particle size) [20]. Smaller NCC particles yield films with a smaller pitch. Desulfation was also found to reduce the chiral nematic pitch [20].
The microstructure of solid NCC films depends on the drying conditions [22]. Suspensions evaporated at ambient conditions generally produce films with polydomain structures in which the helical axes of different chiral nematic domains point in different directions. Drying NCC suspensions in a strong (2 T) magnetic field will align the axes to produce a more uniform texture, increasing the intensity of the iridescence without changing the wavelength [20, 23].
In the laboratory-scale procedure for producing NCC, sonication is used as a final step following acid removal by dialysis, in order to disperse the particles to obtain a colloidal suspension [13, 23]. The effects of sonication on NCC suspension properties have been studied by Dong et al. [14]. They found that brief sonication was sufficient to disperse the cellulose particles and further sonication was counterproductive. A more recent study corroborates this observation [24]. Sonication is thought to break up side-by-side NCC aggregates in suspension [7].
Because particles with larger aspect ratios have smaller critical concentrations for liquid crystal phase formation, increasing sonication has been found to decrease the volume fraction of chiral nematic phase in NCC suspensions of equal concentration. Interestingly, however, sonication continues to affect the critical concentration beyond the point where the NCC particle size is affected [14]. FIG. 2 shows the effect of sonication on mean NCC particle size measured by PCS, for a 15-mL samples of 1.5% (w/w) redispersed freeze-dried sodium-form NCC in 10 mM NaCl, sonicated at 60% output (8 watts) in 4-s pulses with 4-s intervals between. In FIG. 2, it can be seen that the apparent NCC particle size no longer diminishes above 200 J energy input from sonication.
Films of NCC with high uniaxial orientation have also been produced by spinning NCC suspensions derived from the cell wall of a green alga in a rotating horizontal cylinder to produce a gel layer which is subsequently dried [25], but they do not display iridescence. Films of NCC have also been prepared on substrates such as silicon [26]. These films are much thinner than the solid NCC films and are composed of alternating layers of NCC and a cationic polymer (poly(allylamine hydrochloride)). Above a certain thickness, the films exhibit colours that change with increasing thickness, but these colours are due to destructive interference between light reflected from the air-film interface and from the film-substrate interface [26]. Interference colours have also been seen in polyelectrolyte multilayers of microfibrillated cellulose [27]. In addition, films of closely related chitin crystallites retaining the chiral nematic order present in aqueous suspensions above a critical concentration have been produced [28].
There is no known method to shift the iridescence wavelength of solid NCC films that contain no additives. In addition, there has been no method to shift the iridescence wavelength of solid NCC films in the direction of the red end of the visible electromagnetic spectrum.